The present invention relates to a dispersing stabilizer used in suspension polymerization, emulsion polymerization or postemulsification. More particularly, the invention relates to a dispersing stabilizer which enables to carry out a polymerization of vinyl chloride or a mixture of vinyl chloride and other copolymerizable monomers is good workability and which can provide vinyl chloride polymers having excellent quality.
Conventionally, a suspension polymerization method wherein a vinyl chloride monomer is dispersed in an aqueous medium in the presence of a dispersing stabilizer and polymerized using an oil-soluble catalyst is widely used for industrial production of vinyl chloride polymers. In general, the primary factors controlling the quality of vinyl chloride polymers are polymerization conversion, water/monomer ratio, polymerization temperature, amount of catalyst, kind and amount of dispersing stabilizer, and the like. It is a general knowledge that among them, the influence of the dispersing stabilizer is greatest.
A polyvinyl alcohol resin having a specific degree of hydrolysis and a specific degree of polymerization can be used as the dispersing stabilizer, properties of the vinyl chloride polymers, such as plasticizer absorption property, fish eye and particle size can be controlled thereby.
Various polyvinyl alcohols suitable as the dispersing stabilizer or suspension polymerization method for the production of vinyl chloride polymers using such dispersing stabilizers are proposed, for example, a modified polyvinyl alcohol having a degree of hydrolysis of 60 to 90% by mole and containing 0.01 to 5% by mole of a hydrophobic group and 0.02 to 10% by mole of an anionic hydrophilic group in its side chains (Japanese Patent Publication Kokai No. 56-55403); a partially hydrolyzed polyvinyl alcohol having a degree of polymerization of at most 1,500 and a degree of hydrolysis of at most 90% by mole and containing carbonyl group and vinylene group adjacent thereto in its molecule (Japanese Patent Publication Kokai No. 51-45189); a polyvinyl alcohol resin containing carbonyl group and vinylene group adjacent thereto in its molecule and having a specific block character for the residual acetic acid group (Japanese Patent Publication Kokai No. 8-283313); a polyvinyl alcohol containing carbonyl group and vinylene group adjacent thereto in its molecule and having a specified cloud point (Japanese Patent Publication Kokai No. 5-105702); a polyvinyl alcohol having a specific ratio of weight average molecular weight to number average molecular weight and containing carbonyl group and vinylene group adjacent thereto in its molecule (Japanese Patent Publication Kokai No. 61-108602); and a dispersing stabilizer comprising a vinyl ester resin containing an oxyalkylene group in its side chains, or comprising the vinyl ester resin and a water-soluble high molecular weight material (Japanese Patent Publication Kokai No. 60-58402).
Recently, an ability to remove polymerization reaction heats from the polymerization vessel is greatly improved by improvement in structure of a jacket of a polymerization vessel and progress of operation technique for a reflux condenser. Also, temperature elevation time can be shortened by a hot water charging method, namely by charging a polymerization vessel with an aqueous medium which has been previously heated to a predetermined temperature. Thus, even in a large-sized polymerization vessel having a volume of at least 40 m.sup.3 as used in an industrial production, it is possible to practice a high-speed polymerization which is completed in 6 hours.
However, known dispersing stabilizers have defects or are not satisfactory for use in high-speed polymerization of vinyl chloride or mixtures of vinyl chloride and other copolymerizable vinyl monomers. The dispersing stabilizer proposed in Japanese Patent Publication Kokai No. 56-55403 has the defects that adhesion of scale remarkably occurs owing to strong foaming during suspension polymerization of the vinyl chloride monomer and that although the modified polyvinyl alcohol is advantageous for the hot water charging because the cloud point can be adjusted by introduction of the anionic hydrophilic group, it does not satisfactory serve as protective colloid to oil droplets of monomer, so suspension polymerization of the vinyl chloride monomer may become unstable. As a result, coarse particles of vinyl chloride polymers may be produced and fish eyes may be generated.
The dispersing stabilizers proposed in Japanese Patent Publications Kokai No. 51-45189, No. 8-283313, No. 5-105702 and No. 61-108602 have the defects that since water-insoluble components are generated in the dispersing stabilizers and cause clogging of a strainer provided between a dissolving bath and a reaction vessel, productivity and workability are lowered, and that it is difficult to arbitrarily controll the cloud point of the polyvinyl alcohol polymer.
The dispersing stabilizer proposed in Japanese Patent Publication Kokai No. 60-58402 has the defect that there is a case where the cloud point cannot be properly adjusted, for instance, depending on the kind of modifier, e.g. ethylene oxide, and the amount of modifier for the polyvinyl alcohol polymer, particles of vinyl chloride polymers prepared by suspension polymerization may become remarkably coarse and large particles.
Also, in case of conventional dispersing stabilizers for suspension polymerization comprising a non-modified polyvinyl alcohol polymer, it is necessary to give consideration to conditions such as temperature of hot water to be charged into a reaction vessel or the cloud point of the non-modified polyvinyl alcohol polymer. Accordingly, such dispersing stabilizers may hinder the high-speed suspension polymerization of vinyl chloride monomers, and improvement thereof has been desired.
An object of the present invention is to provide a dispersing stabilizer suitable for use in suspension or emulsion polymerization of vinyl monomers such as vinyl chloride and mixtures of vinyl chloride and other copolymerizable vinyl monomers, or post emulsification of vinyl polymers such as vinyl chloride homopolymer and copolymers.
A further object of the present invention is to provide a dispersing stabilizer suitable for suspension polymerization of vinyl monomers such as vinyl chloride and mixtures of vinyl chloride and other copolymerizable vinyl monomers, which enables to carry out the polymerization in a good workability and can produce vinyl polymers such as vinyl chloride polymers having excellent quality.
A still furhter object of the present invention is to provide a dispersing stabilizer suitable for suspension polymerization of the vinyl monomers, which does not cause adhesion of scale owing to foaming during suspension polymerization, which is applicable to a hot water charging polymerization method without any restriction in the temperature of hot water to be charged in suspension polymerization and the manner of charging the dispersing agent, which can produce vinyl polymers such as vinyl chloride polymers which do not contain coarse particles, have good properties such as plasticizer absorption property and give molded articles having no fish eye.
These and other objects of the present invention will become apparent from the description hereinafter.